Important Points in Springing.
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THE problem of springing is now receiving considerably more attention than was given to it in the early days of commercial vehicle manufacture. Even now the. springs fitted by different makers of vehic!es designed to carry the same loads vary extensively, and many of them are considerably overstressed. Many chassis are designed as if the Trings were added as an afterthought, is their length and other dimensions, inItead of being designed to give the beat 7fiu1ts, have to conform to restrictions mposed by the situation•of the various ;hassis components, and then the spring nakers are blamed if their products cause nore trouble than any other part of the !bassis. Few spring makers are at all :content with the limitations imposed upon them by the manufacturers of the hassle, and they are still more unhappy vhen the springs are weakened further .3r boles being drilled through them for pring bolts, etc. Springs act in a very similar way to he ordinary pendulum, and their flexi
bility depends upon the number of complete oscillations per minute which they will give when under load. This number can be determined by the formula : 187.5
= —
d where d is the deflection in inches.
To give ideal springing d should be approximately 5 ins., giving a period of 84, but even in touring cars this figure is seldom, if ever, achieved, and a deflection of between 3 ins. and 4 ins, is considered passable, giving periods of from 93.7 to 100 oscillations per minute.
The bouncing or vertical vibration period must be as stow as possible within practical limits, -whilst the rolling or side vibration period must be as fast as possible. By having a low-built cha.ssis and light body the period of rolling vibration will be high and, therefore, satisfactory', whereas with a high chassis and heavy body the rolling period will be low and unsatisfactory. This is one
of the difficulties which has to be contended with in the design of vehicles, such as motor omnibuses, in which the centre of gravity is high compared with ordinary lorries. In considering this difficulty, the body and chassis may be regarded as a pendulum with a centre of gravity as the bob.
Taking the work stored up in a spring as equal to the weight imposed upon it, multiplied by one-half the initial deflection in feet, then no more than 5 ft.-lb. of work should be stored in each 1 lb, of spring, and it is thus a simple matter to estimate the weight of the spring required for any load. With this figure of 5 ft.-lb. the spring must be stressed uniformly, otherwise it will not be flexible, and the coachbuilder will be tempted to extract plates in order to make the spring more so. In general, end with laminated springs, by doubling the width and length, the strength is doubled, but the weight is quadrupled. Doubling the length alone quadruples the deflection, and thus we can appreciate the benefit of using long springs. .
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